Remote-sensing of diatom bloom succession

Marine diatom blooms are crucial to the oceanic carbon cycle as they represent a major component of the carbon export budget and sustain many fishery-supporting ecosystems. These attributes are strongly associated with the succession of the bloom: the transfer of the carbon up the food chain occurs during the early/mid stages of the bloom, while the flux of carbon export intensifies as the bloom fades, diatoms senesce, and the recycling microbial community takes over. The goal of this work is to detect blooms of large phytoplankton cells (such as diatoms) in the open ocean and determine the stage of the bloom using remote-sensing reflectance (R_rs). R_rs collected aboard a Lagrangian float during the North Atlantic Bloom Experiment 2008 (NAB08) was used in combination with in-situ measurements of phytoplankton pigments from nearby CTD casts. The optical community index for bloom succession (Cetinic et al. 2015), originally determined using the ratio between chlorophyll fluorescence and particulate backscatter (b_bp), was applied here using the ratio between chlorophyll concentration (Chl) from R_rs (OC4 algorithm) and R_rs at 645 nm (proxy for b_bp due to the low absorption in this band). Similarly to the previous study, the Chl/R_rs645 ratio showed highest values associated with the diatom bloom peak, intermediate values associated with early stages of the bloom, and lowest values associated with low phytoplankton biomass of pico- and nano-sized cells. When applied to satellite imagery, the Chl/R_rs645 ratio allowed the assessment of the temporal and spatial distribution of bloom patches in different succession stages. In addition, the correlation between Chl and R_rs in different parts of the visible spectrum allowed the differentiation between absorption- and backscatter-dominated areas. In absorption-dominated sites, the contribution of different phytoplankton taxa was estimated by assessing hyperspectral R_rs features that co-vary with the chlorophyll concentration. Although current satellites do not measure R_rs hyperspectrally, future missions such as NASA’s Plankton, Aerosol, Cloud, and ocean Ecosystem will allow the use of these tools to locate remote diatom blooms in space and time, and potentially aid the assessment of the consequences of these blooms to biogeochemical cycles and the dynamics of food webs.